MRI is an imaging method which magnetically excites nuclear spins of an object set in a static magnetic field with an RF pulse having the Larmor frequency and reconstructs an image based on MR signals generated due to the excitation.
The aforementioned MRI means magnetic resonance imaging, the RF pulse means a radio frequency pulse, and the MR signal means a nuclear magnetic resonance signal.
In MRI, EPI (Echo Planar Imaging) is known as an imaging method. EPI is a high speed imaging method that involves consecutively inverting a gradient magnetic field at high speed for each nuclear magnetic excitation to cause consecutive echoes, thereby acquiring MR signals.
More specifically, according to EPI, all the data required for image reconstruction are acquired by generating consecutive gradient echoes by phase encode steps after applying an excitation pulse and before the magnetization in the xy plane attenuates and disappears because of transverse relaxation (T2 relaxation).
EPI can be sorted into SE EPI, which is based on a spin echo (SE) method and acquires a spin echo signal that occurs following an excitation pulse and a refocusing pulse, FE EPI, which is based on a field echo (FE) method and acquires an echo signal that occurs following an excitation pulse, and an FFE EPI, which is based on a fast FE method.
A type of EPI that generates image data on one image by combining data on an echo train obtained by applying a plurality of excitation pulses is referred to as multi-shot EPI, whereas a type of EPI that reconstructs an image by one application of one excitation pulse is referred to as single-shot EPI.
An EPI sequence involves high speed inversions of the gradient magnetic field during imaging, and therefore, the acquired echo data contains a phase error. As a result, the image reconstructed based on the echo data is distorted. There are two possible primary causes of the phase error: one is the non-uniformity of the static magnetic field, and the other is the eddy magnetic field caused by the switching of the gradient magnetic field (see Non-Patent Documents 1 and 2, for example).
The phase error caused by the non-uniformity of the static magnetic field described above varies depending on the signal intensity distribution of the imaging target and the spatial distribution of the non-uniformity of the static magnetic field in the imaging region. On the other hand, the phase error caused by the eddy magnetic field described above primarily has a first-order gradient in the readout direction in the real space and inverts the direction of the phase gradient depending on the polarity of the gradient magnetic field in the readout direction during acquisition of echo data.
A conventional technique of reducing the phase error caused by the non-uniformity of the static magnetic field is the method of correcting the non-uniformity of the static magnetic field described in Patent Document 1.
A known method of reducing the phase error due to the causes other than the non-uniformity of the static magnetic field is the technique described in Patent Document 2.
Specifically, according to Patent Document 2, two template shots A and B in which the polarities of the gradient magnetic fields in the readout direction are opposite are performed to acquire echo data before the main imaging. The pair of echo data, acquired by the template shots A and B are the same in echo time and therefore in phase error caused by the non-uniformity of the static magnetic field. Based on this, the phase error component caused by the non-uniformity of the static magnetic field is removed. In this way, the phase error component due to the causes other than the non-uniformity of the static magnetic field is selectively extracted and used as phase correction data.
Patent Document 1 referred to above is Japanese Patent Application Laid-open (KOKAI) Publication No. 2006-255046.
Patent Document 2 referred to above is Japanese Patent Application Laid-open (KOKAI) Publication No. 09-276243.
Nonpatent Document 1 referred to above is Self-Correcting EPI Reconstruction Algorithm; A. Jesmanowicz. et. al.; Proceedings of SMR, No. 619, 1995.
Nonpatent Document 2 referred to above is Phase Correction for EPI Using Internal Reference Line; A. Jesmanowicz. et. al.; Proceedings of SMR, No. 1239, 1995.
As described above, there are various methods of correcting the phase error component in the echo data caused by the non-uniformity of the static magnetic field and the phase error component due to the other causes. And the conventional EPI techniques can provide practically satisfactory images. However, the image distortion caused by the phase error is preferably to be made as small as possible.
A task to be solved by the present exemplary embodiments is to provide a technique different from prior arts for further reducing an image distortion caused by a phase error in EPI.
In one embodiment, an MRI Apparatus is capable of performing EPI that includes transmitting an excitation pulse to cause a nuclear magnetic resonance in an object in a static magnetic field, acquiring a plurality of echo signals generated by repeatedly inverting a polarity of a gradient magnetic field in a readout direction, and reconstructing image data on the object based on the plurality of echo signals. This MRI apparatus includes a first acquisition unit, a second acquisition unit and a correction unit.
The first acquisition unit acquires, as first template data, the plurality of echo signals generated by performing an echo signal acquisition sequence of EPI including application of a gradient magnetic field in a phase encoding direction.
The second acquisition unit acquires, as second template data, the plurality of echo signals generated by performing an echo signal acquisition sequence of EPI including application of a gradient magnetic field in the phase encoding direction after acquisition of the first template data, so that start timing of application of the gradient magnetic field in the readout direction in acquisition of the second template data is shifted from start timing of application of the gradient magnetic field in the readout direction in acquisition of the first template data.
The correction unit performs at least correction of phase error in the echo signals by using the first template data and the second template data.
According to the MRI apparatus having the aforementioned configuration, an image distortion caused by a phase error in EPI can be further reduced by a technique different from prior arts.
According to one embodiment, a magnetic resonance imaging method contains EPI and includes the following steps.
One of the steps is acquiring, as first template data, the plurality of echo signals generated by performing an echo signal acquisition sequence of EPI including application of a gradient magnetic field in a phase encoding direction.
Another of the steps is acquiring, as second template data, the plurality of echo signals generated by performing an echo signal acquisition sequence of EPI including application of a gradient magnetic field in the phase encoding direction after acquisition of the first template data, so that start timing of application of the gradient magnetic field in the readout direction in acquisition of the second template data is shifted from start timing of application of the gradient magnetic field in the readout direction in acquisition of the first template data.
Another of the steps is performing at least correction of phase error in the echo signals by using the first template data and the second template data.
According to the MRI method having the aforementioned configuration, an image distortion caused by a phase error in EPI can be further reduced by a technique different from prior arts.